Adaptive antenna arrays have been well known for years. Instead of making use of a single antenna to transmit or receive a signal, multiple antenna elements are used that are arranged in some geometrical order. This arrangement is typically referred to as an antenna array. For transmission, a signal to be transmitted is presented to all antenna elements of the antenna array. By carefully controlling the amplitude and phase of the signal presented to each antenna, the radiation pattern of the array is influenced. This is achieved because the radiated signals of all antenna elements overlap in the far field, leading to constructive or destructive interference depending on their phase. Likewise, signals received at the antenna elements are superimposed after adapting the phase and amplitude to adapt the reception pattern of the array.
The main advantage of adaptive antenna arrays is that antenna patterns can be formed electronically. One possible application is the so-called beam forming, i.e., creating patterns with a high gain towards a specific direction. By controlling the signal phases at the individual antennas, the beam can be steered towards a target receiver or transmitter and it can also be used to track the target.
Large scale antenna systems (LSAS) are seen as a means for increasing spectral efficiency in upcoming 5G cellular networks. An introduction can be found in “4G Americas' Recommendation on 5G Requirements and Solutions, http://www.4gamericas.org/doc uments/4G%20Americas%20Recommendations%20on%205G%20Requirements%20and%20So lutions_10%2014%202014-FINALx.pdf”.
Antenna configurations with two or more antennas are called Multiple Input Multiple Output (MIMO). In massive Multiple Input Multiple Output (MIMO) systems, very large numbers of antennas are employed at the base station. This number may be larger than the number of active users in the cell or devices in the “internet of things”. The antennas can be used in transmit or receive direction. Using duplex filters, the antennas can be used to transmit and receive, simultaneously.
Massive MIMO systems are an evolution step from active antenna systems. In the 4th generation (4G) active antennas typically contain up to 16 antenna elements each of which may have its own power amplifier. In a massive MIMO system, the number of antenna elements may be much larger, and parts of the signal processing, which in conventional systems is performed at the base stations, may be shifted to the massive MIMO antenna.
FIG. 1 shows a simplified drawing of a conventional architecture for active antennas. Each antenna element 1 is connected to a radio-subunit 2.
For simple active antennas, the radio-subunit 2 may consist of a duplex filter and a phase shifter. In the most advanced and flexible approach the phase shifting is done in the central hub in the digital baseband domain. In this case, the radio-subunit 2 consists of digital-to-analog converters and a transceiver, power amplifiers and filters. The radio-subunits 2 are all connected to a central hub 13. When beamforming is performed digitally, the central hub 13 has multiple tasks:                a) perform Rx beamforming,        b) perform Tx beamforming,        c) perform calibration of amplitude, phases, and sample latencies among the radio-subunits 2, and        d) distribute clock to the radio-subunits 2.        
Digital beamforming possesses the advantage that multiple beams can be formed simultaneously. That is, in receive direction, all Rx signals are individually weighted and then added 15 together to produce a combined signal. This is illustrated in FIG. 2 for an antenna system employing 64 receive antennas. In this antenna system, the 64 receive signals coming from the 64 transceivers are each individually weighted, by multiplying 18 each signal j, 0≤j≤63, with a complex weight rxbf[i,j]. The weighted signals are then added 15 together to form the receive signal rx_signal[j]. In the digital domain, this procedure can be carried out in parallel using different weights to calculate multiple weighted rx_signals i.
In Tx direction, the same signal tx_signal[i] is distributed to branches that connect to the antenna elements 1. In each branch j, the tx_signal[i] 16 is multiplied 18 with a complex weight txbf[i,j]. In the digital domain, this procedure can be carried out in parallel using different weights to calculate multiple weighted tx_signals i to form multiple beams at the same time. This is illustrated in FIG. 3.
The main disadvantage of solutions using the conventional architecture is primarily the missing scalability. The conventional architecture is based on a central hub 13 as shown in FIG. 1 to perform signal processing and to distribute the processed signals. This conventional architecture is limited in performance and scalability due to the following reasons:                Pins: Each radio-subunit needs to be connected to the central hub. The more radio-subunits are used, the more connections are needed. Typically, a central hub is based on a field programmable gate array (FPGA). The number of pins an FPGA provides is limited. Thus, if the number of connections requires more connections than the FPGA provides, a different design is needed.        Scalability: It may be desired that the dimensions of the antenna array (number of radio-subunits) should be adapted to a particular use case. Using the same central hub which is preferred in order to reduce engineering and production costs, for each configuration results in the using an oversized hub for smaller antenna arrays.        Computational power of the central hub: The computational requirements of the central hub scale with the number of antenna elements. For large scale antenna systems (LSAS), the computational complexity may as well exceed the computational complexity that the FPGA provides.        Cost: Even if the computational complexity for the central hub increases linearly with the number of antenna elements, the cost for the FPGA does not. Larger FPGAs tend to be more expensive than two smaller FPGAs with the same combined number of logic elements.        Power consumption and heat dissipation: Not only the required FPGA size increases with the number of antenna elements, but also the power consumption due to the increased number of computations required. Using one central FPGA, heat is generated at a concentrated heat source, which complicates the precautions to be taken for cooling.        Cable and wiring: For a LSAS, if a central hub is used, wiring and connections become a challenge. The cables need to become longer to connect the antenna elements that are furthest away from the central hub. As cables get longer, the challenge to guarantee signal integrity such as slew rate and signal strength increases.        Calibration of the antenna elements: Calibrating the amplitude, phases and sample latencies is necessary for controlling the transmitted and received signals at antenna elements and among the radio-subunits.        
So far, only parts of the necessary calibration procedures like the correction of amplitude and phases among the antenna elements were considered in the prior art. WO2010060953 A1 considers the generation of a PN calibration sequence as well as correlation for estimation of phase and amplitude. But the estimation of a delay and correction of this delay is not presented, similarly to U.S. Pat. No. 8,374,826 B2.
EP 2044784 B1 discloses a very basic architecture of a remote base station via fiber to an antenna array system whereas all radio-subunits are connected to a central hub. An unlimited scalability is not possible with that configuration.
Another drawback in the prior art is the inconvenience of active antenna maintenance.
An active antenna for digital wireless communication systems such as UMTS, LTE, etc. consists of four major functional parts: the digital signal processing, the analog (RF) signal processing, the interface to the base station and the array of antenna elements.
Compared to a conventional analog antenna system, the integration of active components may result in higher failure rates (reduced mean time between failure (MTBF)) and, hence, in increased efforts required to maintain and repair the device. On the other hand, the combination of digital and analog (RF) signal processing and an array antenna provides additional features and degrees of freedom, given that the device can be configured through an appropriate data interface. The maintenance process typically has the following stages:                1. Monitoring the proper function of the device and detecting errors;        2. Ad-hoc measures for mitigating the impact of an error occurred;        3. Repair or replace a defect device;        4. Reintegrate the device to reestablish original network functions.        
One may distinguish the repair of hardware components, which requires physical access to the device from repair of software components (or updates), which can be done remotely through an appropriate data interface. Monitoring of hardware components may be done remotely as well, provided that appropriate measurements can be taken and that information on these measurements can be exchanged through an appropriate data interface. Once a problem occurs, the state of the art assumes antennas to be monolithic blocks. As a consequence, a technician either needs to identify a particular defect and repair it on site, or the complete antenna has to be replaced entirely. The task of the innovation is to reduce the efforts required for repairing a malfunctioning device. This concerns the localization of the malfunctioning component within the antenna system, and replacing the malfunctioning component on site.
It has not yet succeeded in the prior art to overcome the following drawbacks, namely that errors can be localized easily. Often erroneous components, such as FPGAs, power amplifiers, etc. cannot be replaced on site. Thus, the antenna has to be replaced, and the defect antenna needs to be repaired in a workshop that has the appropriate tools available.
But an active antenna may consist out of many modules. In case one module fails, it is desirable to replace a single unit (on site) instead of having to replace the entire active antenna. The units are connected through interfaces. In prior art, the active antenna array is either built as a monolithic structure or out of several units that are connected.
For example, in WO2013123907 A1 and WO2013123913 A1 a modular active antenna is described where each module is contained in an individual radome. WO2013112443 A1 also describes a modular antenna but does not provide a solution for maintenance of the modular array. Easy installation may be more complicated due to the additional wire used as a calibration antenna. U.S. Pat. No. 8,760,353 B2 also uses pieces to have an array that can be maintained easily. However, the solution does not need any connection between the modules. Thus, it only works if the processing is done centrally. WO 2014048350 A1 discloses an antenna which is easy to maintain and install. However, the antenna array as such is not build in a modular way. EP2713436 A1 or WO2013026204 A1 describes a modular structure where active and passive components are built on different modules which can be replaced separately. This approach has the disadvantage that logic elements cannot be separated and that long wires are required to connect the active and the passive components. WO 2013091581 A1 presents a modular antenna array in chain architecture. The modules are not directly connected. Thus, in order to remove any particular module, one would have to disconnect the individual modules. EP 2270923 B1 considers the calibration of a modular array. But it does not know a solution to replace individual modules without having to dissemble the entire antenna, especially, if any module in the center is the one to be replaced.